The present invention relates to an implant for replacing vertebrae, the implant comprised of a central load bearing member having at either end a support element with a contact surface wherein an angle of the contact surface relative to the load bearing member is adjustable to fit a neighboring vertebra.
When in the human spine individual or multiple bone elements (vertebrae) can no longer fulfill their support function due to the presence of tumors or the action of force, they must be removed surgically and replaced with implants. Suitable implants must take over the mechanical task of supporting the neighboring vertebrae. For this purpose, the replacement elements are inserted between the intact vertebrae and are attached to them.
When a single vertebra must be replaced, it is sufficient to insert an implant that is straight with respect to the spinal axis. However, when it is necessary to replace a plurality of vertebrae, for example, when an expansive tumor has to be removed, the curvature of the spine must be considered for the support implant. Depending on the location within the spine (cervical vertebral column, thorax vertebral column, or lumbar vertebral column) the kyphosis or lordosis must be continued within the implant.
From British patent 1 243 353 an embodiment is known in which the implant is comprised of modular elements which are screwed together and which are fixed relative to one another with a curved splint that is provided with slotted holes such that the assembled implant corresponds to the desired curvature of the vertebral column. The assembly of the individual elements to form an implant requires time consuming manipulations that must be performed during surgery.
A simple implant is known from German patent 36 37 314 which is comprised of a cylinder-shaped rolled grid element. This known implant, however, must be filled, especially when replacing a plurality of vertebrae, with a bone cement in order to prevent bending of the implant. Ingrowing bone material would not be able to perform the support function because its supporting action would be insufficient.
Another implant is known from U.S. Pat. No. 4,932,975 which comprises a rigid central load bearing member having attached at either end a support element with intermediately positioned elastic members. The elastic members allow for a slanting of the support elements relative to the main axis so that their contact surfaces can be adapted to the angular position of the abutment surfaces of the neighboring vertebrae. The restoring force of the elastic members, however, act continuously as a non-physiological load on the neighboring vertebrae.
The above described disadvantage can be avoided with an implant according to German Gebrauchsmuster 91 07 494 which is comprised of a threaded rod with oppositely oriented threads as a central load bearing member whereby the ends of the threaded rod are screwed into a ball joint which is supported in a support element. The slanted arrangement of the support elements is achieved in this embodiment without spring force. The thread system, however, does not allow for an adaptation to the curvature of the vertebral column.
It is therefore an object of the present invention to provide an implant of the aforementioned kind that can be implanted with simple manipulations and is adaptable without exerting undesirable force loading to the anatomical conditions of the spinal column in order to replace one or more vertebrae.